1. Technical Field
The present invention relates to a method for manufacturing a complicated fiber grating by probing a diffraction of a phase mask. More particularly, the present invention relates to a method for manufacturing a long-length fiber Bragg grating (FBG) having a complicated structure, and a method for manufacturing a sequentially joined fiber grating longer than a phase mask in order to reduce errors in manufacturing fiber gratings. The present invention further provides a method for sequentially joining a plurality of grating sections into a fiber grating longer than a phase mask.
2. Description of Related Art
In recent years, many methods have been proposed for manufacturing a long-length fiber grating having a complicated structure, including the moving fiber-scanning beam technique and the sequential writing technique. When these two techniques are applied to a UV writing process, a He—Ne laser interferometer is required to track the position of an optical fiber. However, while manufacturing a long-length fiber grating, an accumulative error in the fiber position may occur during exposure due to a drifting of fringes generated by the interferometer and an inaccurate estimation of grating periods. This error has great impact on the manufacturing process and adds to the difficulties in manufacturing fiber Bragg gratings having complicated refractive-index changes and phase shift structures.
A patent application was filed in Taiwan by the present inventors of a method for manufacturing a fiber Bragg grating (Taiwan Patent Application No. 094115916; filed on May 17, 2005), wherein a phase distribution of a reference fiber grating is probed before each overlapping UV exposure. In this method, a feedback system is employed to probe a phase distribution of the reference grating at each locating point, so that the phase distributions of the reference grating before and after the reference grating is moved can be controlled to be consistent, ensuring that a phase of each written grating section is continuous from a phase of a previous grating section. However, the reference grating has a very small fiber core radius and a low first-order diffraction efficiency, making it difficult to calibrate the accuracy of position monitoring.
Furthermore, it is known from numerical simulation results that, for a subsequent fiber grating to have a frequency spectrum of a desired quality, a period mismatch between a period of the grating used for position monitoring and a period of UV fringe writing must be smaller than 5%. For ease of contrast, the prior art techniques mentioned in the first paragraph of this section are summarized below in comparison with the present invention to demonstrate their respective pros and cons.
(1) Taiwan Patent No. 434431
According to the method of this patent, a light beam is used to directly write a grating into a moving waveguide. Without using a phase mask and the interference technique, the grating is written in a point-by-point manner, whose refractive index is changed along the waveguide solely by controlling a moving speed of the waveguide. This method is suitable only for a long-period fiber grating and not applicable to a short-period fiber Bragg grating.
(2) U.S. Pat. Nos. 6,834,977 and 6,813,079
In these two patented methods, a grating is written into an optical fiber through a phase mask, section by section, by exposure to a continuous UV writing beam in an overlapping manner. Besides, an interferometer is used to monitor a position of a translation stage at each locating point. However, using the interferometer to monitor a writing location on the optical fiber leads to an accumulative error and requires accurate calibration of a period of the UV writing beam in advance. In the present invention, a grating is sequentially written in a section-by-section manner by probing a phase of a reference fiber grating, wherein a standard phase distribution is provided as a reference for writing. Therefore, according to the present invention, there are no accumulative error-related problems, and the accuracy of period calibration does not influence the manufacturing process significantly.
(3) U.S. Pat. No. 5,945,261
In this patented method, which utilizes the principle that an optical fiber exposed to a UV light will produce a fluorescent light, a grating section is created in advance by UV exposure and, by probing an intensity of the fluorescent light as a feedback system, a position of a translation stage is adjusted according to the fluorescent reaction of this pre-exposed grating section, so that a very long grating can be sequentially formed without phase discontinuity. And yet this method does not allow arbitrary insertion of phase shifts. According to the present invention, however, an arbitrary phase shift can be easily added to any location.
(4) U.S. Pat. Nos. 6,753,118 and 6,801,689
In the methods of these two patents, a feedback system is provided to compensate for drawbacks associated with gratings written by section-by-section, overlapping exposures. More particularly, a spectral response of a written grating is used to calculate corrections. However, as it is difficult to perform real-time monitoring with these methods, they are more suitable for regenerating written gratings. In contrast, the present invention allows real-time adjustment of phase distributions at each locating point, so that a grating can be completely created by writing with a UV writing beam only once.
(5) U.S. Pat. No. 5,830,622
In this patented method, refractive indices are adjusted by additional UV exposures at predetermined locations to introduce additional phase shifts. Therefore, scanning must be conducted for a second time, which is rather time consuming. In addition, the desired phase shifts to be introduced are hard to obtain in a section-by-section manner. In the present invention, however, an arbitrary phase shift can be easily added to any location without additional UV exposures.
(6) Paper published in Electronics Letters (1995), p. 1490
In this paper, grating writing is performed with a moving optical fiber and a phase mask. A fiber grating created by this method is limited in length by a length of the phase mask, while a resolution of a written pattern is affected by a limit on a moving speed of the optical fiber. In contrast, in the present invention, a grating is sequentially written in a section-by-section manner by probing a phase of a reference fiber grating. Therefore, the grating is not limited in length by a phase mask while a resolution of a written pattern is controlled by a writing time.
(7) Paper published in Journal of Lightwave Technology (1997), p. 1419
In this paper, a grating is also written by overlapping exposures in a section-by-section manner while an interferometer monitors a position of a translation stage at each locating point. However, a UV pulse is used in this paper as a writing beam, and a pulse laser causes additional noise problems. The present invention causes no such problems because a grating is sequentially created in a section-by-section manner by switching on and off a continuous UV writing beam.
(8) Paper published in Applied Optics (2002), p. 1051
In this paper, a grating is also written by overlapping exposures in a section-by-section manner while an interferometer monitors a position of a translation stage at each locating point. Although a continuous UV beam is used as a writing beam, the technique of using the interferometer to monitor a writing location on an optical fiber results in an accumulative error, which requires calibrating a period of the writing beam in advance. In contrast, according to the present invention, a grating is sequentially created in a section-by-section manner by probing a phase of a reference fiber grating, so that there are no accumulative error-related problems.